Symptoms:
[email protected]: host alt1.gmail-smtp-in.l.google.com[64.233.177.27]
said: 450-4.2.1 The user you are trying to contact is receiving mail at a
rate that 450-4.2.1 prevents additional messages from being delivered.
Please resend your 450-4.2.1 message at a later time. If the user is able
to receive mail at that 450-4.2.1 time, your message will be delivered. For
more information, please 450-4.2.1 visit 450 4.2.1
https://support.google.com/mail/?p=ReceivingRate w138si33038971ybg.321 -
gsmtp (in reply to RCPT TO command)

Cause:
The Gmail recipient is accepting mail at a limited rate. All emails above this rate are blocked.

Solution:
Wait until the emails are delivered.
If you’re sending many emails at a time, or emails to large groups, review Gmail bulk senders guidelines.
If the issue persists and receiver should receive email, contact Gmail support at https://support.google.com/mail/.